1. Field of the Invention
The present invention relates to power amplifiers and, more particularly, to power amplifiers used in battery powered mobile handsets, and even more particularly to optimizing battery current use in such power amplifiers over a range of output power levels.
2. Background Information
Radio-frequency (RF) signals generated at a mobile handset generally are amplified, transmitted through the handset antenna and sent to a base station for distribution to receivers. Often the frequency bands of operation of the handsets are predetermined, mainly in the frequency range from 800 MHz to 2000 MHz for various mobile standards such as WCDMA (wide band code division multiple access) and CDMA (code division multiple access). The present invention, however, may find advantageous use in device operating at other frequencies and with other formats.
In general, the handset is required to transmit at a high output power level when it is farther away from a receiving base station in order to maintain a pre-determined signal strength at the base station for sufficient reception. Conversely, the closer the handset to the base station, less transmitted power would be required. The handset output power is adjusted according to the command embedded within the RF control signal transmitted from the base station to the handset.
The handset transmitted signal, and hence the RF power amplifier output signal, has to meet the FCC regulation on spectral re-growth (also known as linearity—often measured in terms of adjacent channel leakage power ratio (ACLR) which stipulates the maximum allowable interference to other frequency channels in order to minimize interference between signals). Some known mobile devices (handsets) have RF power amplifiers powered by the full battery voltage at all times. The RF power amplifies are generally designed to meet the linearity specification at maximum transmit power level (+28 dBm for WCDMA system) under such a bias condition. Statistically, power amplifiers transmits at maximum linear output power only for a small fraction of time, while most of the transmissions take place at a considerably lower power levels (10-20 dB below maximum power). The power added efficiency (PAE, a metric known in the art) of the power amplifiers usually drops off rapidly at backoff or LP (“Low Power” and “back off” are synonymous) levels, resulting in non-optimal current consumption coupled with excessive linearity margin. As known to those skilled in the art, the excessive linearity margin can be a trade-off for lower current consumption in the design of power amplifiers.
The actual output power level from the power amplifier (and hence the handset), is continuous from some −50 dBm to 28 dBm. High Power (HP) and Low Power (LP) modes are known in the art as part of the PA functions. The HP generally applies to the range from 16 dBm to 28 dBm, and LP to power levels below 16 dBm. Use of HP and LP modes further improves efficiency in the low power range as compared to a conventional PA.
The above cross referenced application a) describes power amplifiers that provide a reasonably continuous measurement of output power required from a predetermined optimum relationship, that is defined therein. Also defined in this earlier application is the linearity relationship and adjacent channel interference as determined by the FCC. This earlier application provides the required optimum current consumption at each output power by varying the bias power supply to the power amplifiers. These relationships are well described in the prior art, the incorporated applications and the U.S. patent Kim discussed below. Thus, later references to these relationships are made without detail.
U.S. Pat. No. 6,900,692 B2 to J. Kim et al., (Kim) discloses a system that optimizes the current consumption of the power amplifier at two discrete power output levels, i.e. the maximum output power at LP and HP modes operation. This patent is incorporated herein by reference. Kim describes earlier art, with respect to its invention, that incorporates switches (electronic and/or actual mechanical switches with pole pieces) into the power amplifier design where two different parallel signal paths are utilized, one for the LP and one for the HP mode. However, any such switches are lossy, costly, several must be used to provide bypassing, and complex control may be required. Increased power loss and current consumption and large physical size all limit the applications of such circuits in handset power amplifiers.
Kim's advance over the described prior art provides a two path power amplifier with a low power (LP) and a high power (HP) mode without switches. In the low power mode the high power path is disabled and a bypass circuit carries the low power to the output. Kim describes a circuit that reduces power consumption (and therefore extends battery life) in the low power mode and provides higher PAE (power added efficiency). PAE is a metric term known to those skilled in the art. Kim provides parallel impedance matching/transforming circuits that define two paths to the RFout, one for HP and one for LP operation. Kim's inventive circuitry (See Kim's FIGS. 5 and 6) is reproduced herein as FIG. 1. When high power is required, the Voltage Control 2 will provide a control signal to turn on the PA for signal amplification. When in the low power mode, the Voltage Control 2 will provide a control signal to turn off the PA. Kim's FIG. 8 illustrates a voltage control circuit that provides the on/off control to Kim's driver and power stage.
FIG. 2 details the impedances and power delivered by Kim in the HP and LP modes. In the HP mode, the PA is active and Kim provides an impedance ZintL that is significantly higher 10 than ZintH, so that the power, PH, delivered to the 2nd impedance matching circuit is much higher 22 than the power, PL, delivered to the Ztrans, an impedance transformer. The PA amplifies the signal and provides, via the 3rd and 4th impedance matching circuits, a high power RF out. In the LP mode the opposite occurs, the PA is biased off, and ZintH is significantly higher 12 than ZintL, and PL is much higher 20 than PH. Kim details that the “significantly higher” impedance level means two times (or more) larger.
So Kim discloses and requires the relationships as shown in FIG. 2. That is: a) when in LP mode, ZintL is much lower 12 than ZintH; b) when in HP mode, ZintL is much higher 10 than ZintH; and c) ZintH in LP mode is much higher 14 than ZintH in HP mode, d) ZintL in LP mode is much lower 16 than ZintL in HP mode; and e) ZPA, the input impedance of the PA, in LP mode is much higher 18 than ZPA in HP mode. As evidenced in FIG. 1, Kim, via the voltage control 2, controls the on/off characteristics of the PA depending on the mode of operation. In doing so the impedance characteristics of ZintH and ZintL change due to the change in the input and output impedance level of the PA. So in the HP mode, the PA is biased on and ZintH impedance is much lower than ZintL and most power travels through the PA. In the LP mode, the input and output impedances of the PA change when it is turned off, which increases ZintH and decreases ZintL so that more power travels through Ztrans. Moreover, the change in output impedance of the PA in the LP mode affects the 3rd, 4th and Ztrans impedance. These impedance networks are designed to minimize the power leakage through the 3rd matching circuit to the PA. Implementing these techniques are known to those skilled in the art.
Limitations in Kim can be found directly from FIG. 1. For example, Kim's five impedance matching circuits are complex and several simultaneous matching requirements under HP and LP modes would require tradeoffs in performances. In Kim's HP mode operation, Ztrans is designed in conjunction with the 3rd and 4th impedance networks to increase ZintL. While under LP mode operation, these three impedance matching networks are required to reduce ZintL. If the matching networks are optimized for the best ZintL under LP mode operation, it might not be optimal under HP mode operation which would increase undesired leakage power through Ztrans and degrade HP mode performance. On the other hand, if the matching networks are optimized for the best ZintL under HP mode operation, then ZintL might not be optimal under LP mode operation which could increase the current consumption of the driver stage.
Moreover, Kim describes his power amplifier system where, in the HP mode, the power leakage into Ztrans is minimized, and most of the power from the driver stage will be amplified by the PA. This operation is similar to the conventional power amplifiers where the final stage delivers the output power and consumes majority of the current, and therefore operates at a much higher thermal temperature than the driver stage.
FIG. 3 illustrates typical PAE of a power amplifier operating in a single power mode. It is evident that in the back-off or low power range, PAE is very low. FIG. 4 shows operation of a system with two power level modes, as in Kim and as might be used with the present invention. Here it is clear that PAE is improved in the LP mode.
Reiterating the limitation of the prior art, referencing FIG. 1, the power amplifier, PA, is supposed to be in the off state in the LP mode operation. However, the PA will start to turn on when there is a significant RF power level presented at the PA's input. Conduction of the PA in the LP mode will significantly degrade the linearity (ACLR level) of the output signal due to phase mismatch of signals from the two paths when they are combined at the input of the output matching, 4th Z matching, network. This limits the maximum linear output power capability under LP mode operation. In addition, the impedance presented by the Ztrans matching network to the 1st Z matching network cannot be selected optimally to maximize LP mode efficiency as a result of limited isolation between the two passive matching networks, 2nd Z matching and Ztrans. Ztrans is tradeoff between efficiency and minimizing power leakage to the interstage 2nd Z matching network under LP mode operation. Typical LP mode efficiency as 16 dBm output power is about 19%.
The prior art, generally, remains power inefficient due to potential tradeoffs required for matching impedances for HP and LP modes, and the prior art also employs complex circuitry.